Phytolith assemblages recovered from the middle Eocene Koluel-Kaike Formation in Central Patagonia, southern South America, confirm the presence of herbaceous components, mainly related to mixed ecosystems or savannahs, and subordinate dry forest communities. This new palaeobotanical information reveals a clear transition between previously described subtropical/tropical woodland-dominated communities from the early Eocene Las Flores Formation, and grass-rich ecosystems from the mid-Eocene Sarmiento Formation. Saddle phytoliths recorded in the Koluel-Kaike Formation would attest the presence of C₄ grasses (Chloridoideae) by the middle Eocene of Central Patagonia. These saddle phytoliths is the oldest grassy habits recorded in Central Patagonia, and preceding in at least ∼8 Ma the timing proposed by previous contributions. The early appearance of grass-dominated ecosystems in southern South America, would be related to the worldwide greenhouse to icehouse systems transition initiated during the mid-Paleogene.